The generalized stacking faults of Al, Pd, Pt, Ir, and Rh are investigated by a parametrized tight-binding potential. The stacking-fault energies (SFEs) are calculated to be in good agreement with experimental data, except for Al. More important, it is found that the SFE of Pt may be reduced by 14% by atom relaxation while the effect of atom relaxation on the SFEs of Al, Pd, Ir, and Rh are small. Thus, it is concluded that the effect of atom relaxation on SFE should be important, especially for an alloy system where radii difference between two constituting elements is large.